1. Field of the Invention
The invention relates to an improved fixture and a method for removably and accurately positioning the fixture between a headstock and a tailstock, and in particular to an improved fixture suitable for conveniently loading and unloading large structures to be assembled and that provides an operator with 360 degree access to the improved fixture and the structure. More particularly, the present invention is directed to an improved fixture having a frame and a plurality of position compliance devices, each comprising a movable block having a rotatably mounted locating pin, which enables the fixture to be accurately and repeatably positioned between the headstock and tailstock, and that serves to reduce stress on the fixture and the structure being assembled during rotation, which is undesirable if left unreduced.
2. Background Art
Fixtures or tools have been used for many years to facilitate the assembly of large structures such as a slider assembly for a semi-trailer. Because a structure such as a slider is comprised of many large components which must be precisely assembled for its eventual proper operation as a subframe component of a semi-trailer or tractor-trailer, it is typically assembled predominantly by welding on a special fixture designed to hold such large structures. Traditionally, such a prior art fixture was permanently bolted or otherwise affixed to large headstock and tailstock structures, and the slider had to be assembled and welded on the non-removable fixture. Such an arrangement, while resulting in satisfactory assembly of the slider, had certain disadvantages. One disadvantage has been the difficulty of loading and unloading large structures onto the fixture with overhead cranes or lifting tools, and also that the operator/assembler had access to only one side of the fixture.
Heretofore, others have contemplated a removable fixture to overcome the problems of loading/unloading and operator access inherent when the fixture is non-removably mounted on the headstock/tailstock structures, but until applicants' invention the objective has not been satisfactorily achieved. More particularly, the problem that has remained unsolved has been that of obtaining repeatable and precise alignment of the centerline axis of the fixture between the headstock and tailstock structures. Any misalignment can cause unwanted stresses on the fixture and the structure being assembled, such as during 360 degree rotation of the fixture by the headstock structure for welding a slider. If fixture misalignment exists, the slider can be subjected to twisting stresses, or moments, during such rotation. Even without such misalignment, the structure being assembled is subjected to various less severe stresses during fixture rotation. Thus, in order to eliminate such problems, the fixture had to be realigned each time it was reinserted between the headstock and tailstock structures. Such realignment is inefficient.
The present invention solves the above-noted problems through the addition of at least one alignment compliance device to each side of either the tailstock end or the headstock end of the fixture frame, said compliance device comprising a movable block having a rotatably mounted locating pin, and a stationary pin passing through the movable block for capturing at least one tension device located thereon on each side of the movable block. The movable blocks, the locating pins and the tension devices substantially self-align the fixture each time it is reinserted between the headstock and tailstock structures, and also substantially reduce stresses on the fixture and structure being assembled during rotation.